When an offshore oil or gas field is identified and decisions for exploration and production is taken, it is of importance to reduce the time taken for taking such decision till production facilities are provided and production initiated.
In order to start production, the wells have to be drilled and temporarily completed. Such drilling is made by a separate floating or fixed drilling unit. Thereupon the drilling unit is removed a production unit with complete production facilities are brought on to the side and connected to the completed wells. Until such completion is finalized, production of the hydrocarbons has to be postponed.
In shallow waters and in particular in shallow waters with soft or muddy seabed conditions, the size of the drilling or the production units may be decisive, i.e. that it only possible to float in a floating unit with partly completed topside may be floated in, the remaining part of the required top side has to be installed and built subsequent to completed float in and positioned base structure. Upon completed drilling operation, a part of the drilling facilities has to be removed and substituted by production facilities. Installation of such production facilities is complex, expensive and time consuming, increasing the time lapsed from first discovery till start-up of production.
It has previously been proposed to provide harbour sites for LNG loading at sea that either float or are placed on the ocean bottom. The floating sites have the problem in common that the platforms during drilling and production phase are subjected to movement caused by wave action. If the floating platform is intended to transfer of LNG such movement should also be kept at a minimum since the dynamics put great demands on equipment and safety if the loading takes place side by side.
To reduce the problems associated with the dynamics of the floating bodies during loading operations, it has been proposed to install large, rectangular steel or concrete structures on the seabed, functioning as artificial harbours or as a drilling and/or production facility. Typical water depths are 8-30 metres. This type of large construction is intended to be built away from populated areas and floated in and installed at the intended site, most commonly requiring proper foundation in the form of skirts intended to be forced into the sea bed soil, or intended to be piled.
NO 126927 corresponding to GB 1369915 describes a harbour site comprising a number of units that are afloat or sunk and otherwise constructed for placement on the seabed. Each unit comprises a base, load-carrying structure and moveable wave-breaking elements that can be moved according to need.
U.S. Pat. No. 3,958,426 describe a harbour site comprising a number of units placed apart on the seabed, so that at least one straight mooring location is formed. The units are provided with fenders and wave dampening devices.
Applicants own publication WO 2006/041312 discloses a harbour plant for storage, loading and unloading hydrocarbons such as LNG at sea, the whole content of which hereby being included by the reference. The harbour comprises three units built from steel or concrete, placed on the seabed. The units are placed in sidewise relation in-line. The harbour is configured to dampen the waves, the vessel being intended to lie on the leeward side of the mooring.
Applicants own publication WO 2013/002648 discloses a harbour plant for storage, loading and unloading of hydrocarbon products at sea, comprising a number of units being mutually placed on the seabed so that a harbour plant is formed. The units are placed independently at a given distance apart in sideways direction and having a front surface along which a vessel is intended to be moored, forming passage(s) for parts of the waves, and being configured to dampen a part of the incoming waves while allowing other parts of the waves and current to pass through the harbor plant.
However, a field development drilling operation for drilling and completing the wells requires one type of platform, while production from such wells require different facilities. For floating platforms anchored at the site, a drilling platform may be anchored during drilling operations and replaced by another floating platform with production facilities upon completed drilling operations. If the platform is of a type that is intended to be supported by the sea bed soil, however, such platform may either be complete with drilling and production facilities or the platform may be refurbished at the site, removing at least a part of the drilling facilities and installing the required production facilities, increasing the total costs involved.
In addition, the density, composition, consolidation and topography of sea bed soil may vary significantly for one seabed location to another. For example, the soil in river mouths will often be dominated by soft, muddy soil with a kind of yoghurt texture, while other seabed areas may be influenced or overlapped by hard sandstone, limestone or ancient volcanic rock. This will have direct impact on the load bearing capacity of the seabed soil, and hence the possibility to find a predictable and reliable foundation solution for a seabed structure which shall be resting onto the seabed.
Hence, there exists a requirement for cost-effective, versatile and flexible harbour plant systems that can store different oil related products and bunkering, and are easy to build, maintain and repair, and which can be standardized as far as possible for fabrications and cost reasons, and which can easily be deployed (installed) onto any type of seabed soil.